A smooth register transition is a fundamental aspect of the classically trained singing voice. The purpose of this study is to examine the physiological and acoustical bases of the vocal techniques used by trained female singers to transition across the primo passaggio in triads. Triad recordings from ten female singers at various levels of training were analyzed physiologically with an electroglottograph (EGG) signal and acoustically with spectral analysis. The closing quotient (CQ) measurements obtained from the EGG signal revealed three patterns among the subjects, a decreasing, an increasing, and a stabilized CQ across the register transition. The number of the dominant harmonic decreased across the register transition for the professional singers in contrast to the other subjects who maintained a relatively stable harmonic number. The frequency and amplitude of the dominant harmonic remained relatively stable for all the subjects. Data analysis concluded that the method used by the most experienced singers for smoothing the passaggio involved resonating a lower harmonic across the primo passaggio while maintaining a stable vocal tract and laryngeal adjustment.